No I don't think it's OK to ask someone to change something about themselves when dating. The only exception would be if it's something the person has already initiated the idea first by themselves, then one reminder might (and I only say MIGHT) be OK, but otherwise leave it at that. I would never, ever ask someone to lose weight, quit smoking, etc etc etc (even though I myself am not overweight and do not smoke). My strong feeling is, when dating, if you can't accept the person as they are, then walk away. There are other fish in the sea. People should like other people for who they are, not for what they can be.
Now, I will add that it can take time getting to know the person you're dating so you can discern properly whether the issue that bothers you is something that is actually no big deal in the end (e.g. His ears stick out so much!) or whether it is something which is non-negotiable to you or him (e.g. He doesn't want kids, I do). First impressions are sometimes accurate, sometimes not. If unsure, take the time to find out whether you still think the issue really is an issue after awhile of dating - if it remains a problem to you then walk away, if not then enjoy and see where the relationship takes you! But neither men nor women should ask the person they're dating to change something about themselves. My opinion is simply this: If the person would be the perfect match except for [some issue], then, by definition, they are not the perfect match.